[5] Gallagher began acting at the age of two, modeling for print advertorials in and around New York City, which eventually led to commercial work as an actor.

Later that year, he was cast as Simon Camden in 7th Heaven, which aired for eleven seasons, making it the longest-running family drama on television,[6] and became the highest-rated show on The WB.

After leaving 7th Heaven, he had several roles, including serial killer Buck Winters in Numb3rs, a murder suspect in the CSI: Miami episode "Dangerous Son", Mark in Boogeyman 2, Paul Shapiro in Saving Grace, Ryan Stephenson in Bones, Jeff Ellis in Without a Trace, Seiji Amasawa in the English dub of Whisper of the Heart, and Kevin Mather in The Deep End episode "An Innocent Man".

In March 2012, it was announced that Gallagher would be co-starring with Jake Weber in Scared of the Dark, directed by Takashi Shimizu.

In April 2024, it was announced that Gallagher and his former co-stars, Beverley Mitchell and Mackenzie Rosman, would co-host the 7th Heaven rewatch podcast Catching up with the Camdens!.